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GEU 0047: MeteorologyLecture 11: Air Masses and Fronts
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• the air with similar properties of temperature and moisture which covers a large area• Semi-permanent structures in the atmosphere
Air Masses
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• Moisture content– Dry Continental (c)– Moist Marine (m)
• Temperature– Cold Polar (P), Arctic (A)– Warm Tropical (T),
• Results in 5 basic types– cA Continental Arctic
– cP Continental Polar mP Marine Polar
– cT Continental Tropical mT Marine Tropical–
Air Mass Classifications
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Global air mass source regions
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Air Mass Source Regions
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Source Region Profiles
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• Arctic Air Mass– Bitterly Cold and Usually Very Dry– Develops over the snow or ice covered regions of continents– Similar to polar air masses but usually develops in regions
north of 60N where there is little or no insolation during the winter.
Arctic Air (cA)
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• Continental Polar Air Mass– Cold, Dry– Develops over the interior of
• North America -- Central Canada -- Siberia
Continental Polar (cP)
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• Marine Polar Air Mass– Cold, Moist– Develops over the cold ocean waters of the North
Pacific and North Atlantic
• Usually associated with conversion of cP siberian air to mP which is moistened and warmed from the Japanese “Gulf Stream Current”
• Moves across the N. Pacific to affect N. California and the Pacific NW
• Responsible for fueling “Nor-easters”
Marine Polar (mP)
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• Continental Tropical (cT)– Hot, Dry– Develops over the deserts of Mexico and
the Southwestern United States– Very unstable because of heat and
convection, but cloudless because of lack of moisture.
• Marine Tropical (mT)– Warm, Humid– Develops over the tropical and subtropical
oceans and the Gulf of Mexico
Tropical (T)
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mT
cP
Example Air Masses
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• Air masses do not always remain over their source regions.
• If the air moves over a region that is different from where it originated, the air mass will be modified, or changed, by the land/ocean surface that the air is travelling over.
• Changes include: warming, cooling, adding or reducing moisture content
Air Mass Modification
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cP
WarmerLand
The cP air mass willbe warmed by thewarmer land that itpasses over.
Air Mass Modification
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Air Mass Invasion
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Air Mass Invasion
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Air Mass Invasion
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Cold air mass moves over a warmer surface.
cP
Warm, Moist
The lower layers arewarmed and maybecome unstable.
Changes in Stability
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0
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Altitude(m)
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Sounding Before Modification:Conditionally Unstable
After modification:Absolutely Unstable
Cold air mass moving over a warmer surface.
Dry AdiabatMoist AdiabatEnvironment
Stability Changes
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Warm air mass moves over a colder surface.
mT
Warm, Moist
The lower layers arecooled and tend to
stablize.
Cold
Stability Changes
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Sounding Before Modification:Conditionally Unstable
After modification:Absolutely stable
Warm air mass moves over a colder surface.
Dry AdiabatMoist AdiabatEnvironment
Stability Changes
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Frontal Boundary Symbols
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Fronts
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• Front– A narrow transition zone between air
masses of differing densities.
– The density differences usually arise from temperature differences.
– Density differences may be a result of humidity differences (summer).
• A front is the boundary or transition zone between different air masses.
Fronts
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• The transition zone is usually narrow (15 to 200 km in width).
• Frontal Boundaries may be over 1000 km long.
• On maps, we draw the frontal boundary on the warm side of the transition zone.
Air Mass Invasion
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• Cold Front– A boundary that moves in such a way
that the colder (more dense) air advances and displaces the warmer (less dense) air.
– The largest temperature differences are normally associated with cold fronts.
Idealized Cold Front
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Cross-sectional Characteristics
Cold Air
Warm Air
Strong Lifting near thefrontal boundary can produce convective clouds.
Idealized Cold Front
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Idealized Cold Front
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• Precipitation:– Located on either side of the front.
– Convective, showery in nature
– If warm air is stable• Nimbostratus, Altostratus
– If warm air is unstable• Cumulus, Cumulonimbus, Thunderstorms
Idealized Cold Front
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Weather conditions associated with a cold front
Weather Phenomenon
Prior to the Passing of the Front
Contact with the Front
After the Passing of the Front
Temperature Warm Cooling suddenly Cold and getting colder
Atmospheric Pressure
Decreasing steadilyLeveling off then increasing
Increasing steadily
Winds South to southeast Variable and gusty West to northwest
Precipitation ShowersHeavy rain or snow, hail sometimes
Showers then clearing
CloudsCirrus and cirrostratus changing later to cumulus and cumulonimbus
Cumulus and cumulonimbus
Cumulus
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• Warm Front– A boundary that moves in such a way that
the colder (more dense) air retreats and is replaced by the warmer (less dense) air.
– The warm fronts tend to have weaker temperature gradients.
Idealized Warm Front
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Idealized Warm Front
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Weather conditions associated with a warm front
Weather Phenomenon
Prior to the Passing of the Front
Contact with the Front
After the Passing of the Front
Temperature Cool Warming suddenlyWarmer then leveling off
Atmospheric Pressure
Decreasing steadily Leveling offSlight rise followed by a decrease
Winds South to southeast Variable South to southwest
PrecipitationShowers, snow, sleet or drizzle
Light drizzle None
CloudsCirrus, cirrostratus, altostratus, nimbostratus, and then stratus
Stratus, sometimes cumulonimbus
Clearing with scattered stratus, sometimes scattered cumulonimbus
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• Cold fronts typically move faster than warm fronts.
• Cold fronts can catch up and “overtake” a warm front.
• Depending on the different air masses, there are two types of occlusions:– Cold type occlusion– Warm type occlusion
Occluded Front
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Cold Air
Cool Air
Warm Air
Cold-type Occlusionwhen the air behind the front is colder than the air ahead of the front.
Cold type occlusions are common on the east coast of continents and generally form when the continental polar or arctic air collides with maritime subtropic air.
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Cold AirCool Air
Warm Air
Warm-type OcclusionWhen the air behind the front is warmer than the air ahead of the front
Warm type occlusions are common on the west coast of continents and generally form when maritime polar air collides with continental polar or arctic air.